To the end of achieving a large output in a high speed range and a large torque in a low speed range, a valve actuating device equipped with a variable valve actuation property control device for varying at least one of valve timing and a valve lift primarily according to a rotational speed of the engine is proposed, for instance, in Japanese patent laid open publication No. 63-16111. According to this proposal, a plurality of (for instance, two) valve actuating modes for different output torque properties are provided so that the range of the output property of the engine may be widened by selectively establishing one of the valve actuating modes which is suitable for each particular operating condition of the engine.
In an engine equipped with such a variable valve actuation property control device, it is preferable from the view point of the driveability of the vehicle equipped with the engine to avoid a sudden change in the engine output when switching over between a valve actuation mode suitable for low speed operation and another valve actuation mode suitable for high speed operation, and a technology is disclosed in Japanese patent laid open publication No. 02-042105 to switch over between the two modes when the outputs by the high speed mode and the low speed mode substantially agree with each other.
Meanwhile, if the switch over between two different valve actuation modes is carried out before the engine has been warmed up, unsatisfactory operation of the mechanism for valve actuation mode switch over may develop due to high viscosity of the lubricating oil which is used in the mechanism as a working fluid and irregular combustion resulting from unfavorable atomization of fuel under low intake temperature condition, and a control program is known which detects the warm-up of the engine as a condition for allowing the switch over between different valve actuation modes (refer to FIG. 7 of Japanese patent laid open publication No. 02-042105).
According to this control program, when the vehicle is operating in a range suitable for the high speed mode before the engine is warmed up, the low speed mode is selected until the completion of the warm-up of the engine is detected, and the transition from the low speed mode to the high speed mode occurs immediately after the detection of the engine warm-up. Therefore, particularly when the mode switch over is executed when the engine is warmed up under high load condition (with a large throttle opening), as there is a large difference in the engine output between the high speed mode and the low speed mode, a relatively large shock is expected to occur upon detection of the warm-up of the engine. As this is a change in the engine output which the operator of the vehicle may not be expecting, and substantially impairs the smooth impression of the vehicle, it should be avoided.
It has been proposed, for instance in Japanese patent laid open publication No. 02-157440, to control the slipping of driven wheels of an automobile by detecting the slip ratio of the driven wheels according to the deviation of the rotational speed of the driven wheels from that of the non-driven wheels, and reducing the output torque of the engine by making the air/fuel ratio of the engine intake leaner.
In mass produced engines, it is unavoidable to produce some variations in output properties because of inevitable manufacturing errors in the fuel injection valves and the camshafts and control errors in engine control. Therefore, the data stored in the engine control device serving as the basis for the control commands produced by the engine control device may deviate from the ideal data for engine control. In particular, the error in the air/fuel ratio in making the air/fuel ratio leaner for the purpose of traction control may cause a larger change in the actual engine output than what is considered to be acceptable.
In particular, when the valve actuation mode is to be switched over while such a traction control is being executed, the actual outputs of the high speed mode and the low speed mode may deviate from each other to such an extent that the high speed mode may actually give rise to a lower engine output than the low speed mode. In such a case, since the output torque of the engine is reduced when the valve actuation mode is changed from the low speed mode to the high speed mode, the slip ratio of the driven wheels is decreased upon the mode change, and the air/fuel ratio may be made richer or restored to a normal level so as to terminate the traction control. This in turn causes an increase in the output torque of the engine, and a corresponding increase in the slip ratio of the driven wheels, and produces a condition suitable for the resumption of the traction control, thereby creating an undesirable condition of hunting.